Climate change fingerprints found on this summer's scorching droughts
Why it matters: The study, published Wednesday, found human-caused global warming dramatically upped the odds of drought throughout the hemisphere outside of the tropics — making them at least 20 times more likely compared to the preindustrial era.
- The Northern Hemisphere summer of 2022 brought extreme heat and drought to Europe, China and North America.
- Drought conditions tied to the heat caused water shortages in the Western U.S., led to rivers running critically low in countries like Germany, and caused food prices to increase.
- The drought conditions harmed agricultural output at the same time as unrelated factors, such as the war in Ukraine, were exerting upward pressure on food prices.
The big picture: This past summer featured cascading drought events around the Northern Hemisphere. Hit especially hard were the Western and Central U.S., Western Europe and large swaths of China.
- The new study, published using peer-reviewed methods by the 21 members of the World Weather Attribution group, a global collective of researchers that examine the ties between extreme weather events and human-caused climate change, focused on changes in soil moisture as a key drought metric.
- Warmer temperatures are the primary cause of the increase in drought likelihood, the study finds, since hotter weather increases evaporation from soils and plants.
- The new research estimates that human-caused climate change made soil moisture drought conditions much more likely, by about a factor of 20, though the study's authors cautioned that there is a significant amount of uncertainty associated with that specific number.
- "Our analysis shows that last summer's severe drought conditions across large parts of the Northern Hemisphere were fueled by human-induced climate change," said Dominik Schumacher, of ETH Zurich, in a statement. "With further global warming we can expect stronger and more frequent summer droughts in the future."
What they did: The researchers looked at soil moisture levels during June, July and August for both the top few inches of soil, and the top 1 meter, or 39 inches, which is known as the root zone.
- This is the depth of soil that most plants draw water from, so dry soils there can have a particularly large effect on agriculture.
Zoom in: Hemisphere-wide, the study found human-caused climate change made surface-level drought at least 5 times more likely to occur compared to a climate without added amounts of greenhouse gases. Root zone drought had the 20-fold increase, however.
- The international team of researchers also focused on Western and Central Europe, which saw an especially severe drought from the Atlantic to the eastern Black Sea.
- The study found that for this region, climate change made surface drought up to 6 times more likely to occur, while root zone drought, which is also known as agricultural and ecological drought, was 3 to 4 times more likely.
Yes, but: The reduced spike in the odds of regional drought compared to hemisphere-wide conditions can be traced to the fact that it tends to be easier for researchers to tease out broad trends when looking at bigger regions, with more natural climate variability entering the picture when they zoom in on a specific area.
What they're saying: "There's absolutely no doubt that climate change did play a big role here," said study coauthor Friederike Otto, a climate scientist at Imperial College London, in a press call Wednesday.
- "The exact quantification of that role is more uncertain for soil moisture than, for example, when we look at heavy precipitation," she said.
The bottom line: The agricultural drought seen around the Northern Hemisphere this summer can now be expected to occur once every 20 years, as opposed to about once every 400 years in the preindustrial era.